(1) Field of the Invention
The present invention relates to a process for purifying a nitrogen trifluoride gas. More specifically, it relates to a process for particularly removing oxygen difluoride from a nitrogen trifluoride gas.
(2) Description of the Prior Art
In recent years, a nitrogen trifluoride (NF.sub.3) gas is noticed as a dry etching agent for semiconductors and as a cleaning gas for CVD apparatus, but the nitrogen trifluoride gas for use in these use purposes is required to be pure as highly as possible.
The NF.sub.3 gas can be prepared by a variety of methods such as the so-called molten salt electrolysis method in which NH.sub.4 F.multidot.xHF which is prepared by acidic ammonium fluoride alone or ammonium fluoride and hydrogen fluoride as the starting materials is electrolyzed, or the method in which ammonia and fluorine are reacted. The present inventors have confirmed that NF.sub.3 gases obtained by any of the methods contain oxygen difluoride (OF.sub.2) in most of the cases with a relatively large amount. Particularly, in NF.sub.3 gas obtained by the molten salt electrolysis method, at most several hundred ppm or so (wherein "ppm" represents a volume standard, hereinafter the same) thereof are contained.
It has been known that in the NF.sub.3 gases prepared by the above methods, various compounds such as nitrogen (N.sub.2), dinitrogen difluoride (N.sub.2 F.sub.2), nitrous oxide (N.sub.2 O), carbon dioxide (CO.sub.2) and unreacted hydrogen fluoride (HF) are contained as impurities.
Accordingly, in order to obtain a high purity NF.sub.3 gas, it is required to remove these impurities, and various methods for purification have been proposed. Regarding OF.sub.2, in "Deutsche Luft und Raumfahrt", Forschungsbericht, Oktober 1966, Herstellung von Stickstoff-fluoriden durch Elektrolyse, p. 21, lines 19 to 20, there is described that OF.sub.2 in a NF.sub.3 gas can be reduced by using sodium sulfite and potassium iodide aqueous solutions. Also, in "Fluorine Chemistry and Industry, Advance and Application", edited by Nobuatsu Watanabe (Published by Kagaku Kogyosha), p. 208 (1973), there is described the method that a NF.sub.3 gas is washed with an aqueous sodium thiosulfate solution to remove N.sub.2 O in the NF.sub.3 gas. However, there is not described that OF.sub.2 in the NF.sub.3 gas can be removed.
Also, in J. Massonne, Chem. Ing. Techn. vol. 41(12), p. 695 (1969), there is described, as a method of removing N.sub.2 F.sub.2 in a NF.sub.3 gas, the method in which an aqueous solution such as of KI, HI, Na.sub.2 S, Na.sub.2 S.sub.2 O.sub.4 ; or Na.sub.2 SO.sub.3 is used to remove N.sub.2 F.sub.2 therefrom. However, according to the method, for removing N.sub.2 F.sub.2 completely, a relatively long time is required so that the reaction bath becomes considerably large as well as a large amount of chemicals is required. In this literature, no removal of OF.sub.2 is described.
The present inventors have established the undermentioned quantitative method of OF.sub.2 which uses a low temperature gas chromatography and confirmed that the content of OF.sub.2 in a NF.sub.3 gas sometimes reaches even to several hundred ppm.
Since the OF.sub.2 is an oxygen-containing compound, it can be estimated that it is extremely dangerous in the step of purifying a NF.sub.3 gas. Accordingly, in the case of purifying a NF.sub.3 gas, it is necessary to possibly remove OF.sub.2 at a relatively initial stage.
Also, if OF.sub.2 remains in a NF.sub.3 gas, it is inconvenient for further treating the NF.sub.3 gas to obtain a high purity NF.sub.3 gas, Moreover, if such an oxygen-containing compound is contained, it involves the problem that it exerts bad effect when the NF.sub.3 gas is used as a dry etching agent for semiconductors or as a cleaning gas for CVD apparatus.